This invention relates to a method and apparatus for quenching a metal workpiece by means of jet gas impingement and, more particularly, to a balanced jet gas impingement cooling arrangement which promotes uniform cooling and minimum distortion.
The invention is particularly applicable to a method and apparatus used to quench at a critical cooling rate a super-alloy workpiece having varying thick-thin sections throughout its length and will be described with particular reference thereto. However, the invention is applicable to any configured metal workpiece which must be quenched at a critical cooling rate while being maintained during and after the quench within close dimensional tolerances.
In the heat treatment of metals a wide variety of cooling arrangements have been utilized to achieve uniform cooling of the entire surface of the workpiece which, in turn, results in uniform grain growth and minimal workpiece distortion. When quenching is desired, certain liquid quenches have varied the flow of liquid coolant as between the top surface of the work and the bottom surface of the work, or as between the center of the work and the edge of the work, or to provide additional liquid coolant between adjacent workpieces to compensate for the radiation heat therebetween. In those heat treating applications where the work is to be cooled but not quenches such as where the work is to be furnace cooled or tempered, or normalized, etc., it is known that improved results can be obtained by impinging thick work sections with streams of gas coolant to achieve somewhat similar cooling rates between the thick sections of the workpiece as well as the thin sections of the piece. Additionally, workpieces have been cooled by a gas coolant directed at one part of the workpiece and a liquid coolant at another part to achieve differential hardening of the workpiece. In the other instances, liquid droplets have been interspersed with gas coolant and the resulting mist spray has been impinged against the workpiece to achieve a controlled cooling rate which is more severe than gas cooling but less drastic than liquid quenching.
As a general proposition, quenching of steel workpieces at or in excess of critical cooling rate has heretofore been accomplished by means of a liquid coolant. For definitional purposes, quenching will be defined as rapid cooling of a workpiece which has been heated to a temperature or at above its upper critical temperature limit (i.e. for plain carbon steels the temperature, 1333.degree. F., whereat the steel undergoes a phase transformation, in that body centered cubic crystals change to face centered cubic crystals and for superalloy steels, the temperature whereat Ni.sub.3 (A1, Ti) is precipitated in a face centered cubic crystal, i.e. .gamma.') to a temperature below the "knee" of the workpiece's isothermal transformation or T-T-T curve. Cooling at the critical rate means quenching the workpiece at a cooling rate which is sufficient to cool the workpiece without the workpiece passing through the "knee" of the transformation curve. As noted, when the end user specifications indicate that the workpiece be quenched, liquid quenches either in the form of spray nozzles have been traditionally employed to achieve the desired cooling rate, or when quench cracking tends to occur, salt or oil bath liquid quenches maintained at an elevated temperature have been employed. While many of the liquid cooling arrangements discussed above have attempted to control the manner in which the workpiece is exposed to the liquid coolant to promote uniform cooling of the workpiece, inherent in any liquid coolant arrangement is the vapor barrier which results when the metal contacts the liquid coolant. The presence of the vapor barrier and the attempts to control the size of the barrier during cooling of the workpiece present significant problems if the workpiece tolerances are extremely close.
In applications requiring close workpiece tolerances, entirely different heat treat approaches have had to be resorted to. If the heat treated part was to be used in either a rolling or sliding manner with another part, such as in a gear or cam application, case hardening of only the surface of the part is affected. In extremely tight tolerance applications, plasma arc heating or induction heating has been employed to minimize the machine finishing operations to be subsequently used. In other applications, particularly in the aerospace field, where through hardness and close workpiece tolerances must be obtained, complicated manufacturing processes have had to be employed. Generally, superalloy steels have had to be employed to arrive at a suitable T-T-T curve where a critical cooling rate could be obtained by a moderate quench which would not produce quench cracks while permitting uniform grain structures throughout. Different thick-thin components of the part would be separately formed and heat treated so that large variations in gain size would not occur and thick and thin parts would then be welded together to produce the composite part which would subsequently be annealed or tempered to stress relieve the junctions of the welded part. This, in fact, was the manufacturing process heretofore used in the manufacture of the workpiece illustrated in the preferred embodiment of this invention. The workpiece disclosed is a rotor for use in jet engines. Prior to the present invention, the hub was forged separately and apart from the blades or fins with each fin individually welded to the hub after quenching. The entire rotor was then subjected to a stress relieving heat treatment. This process was necessitated because the rotor blades would deform outside part tolerances if the rotor was formed as a one-piece assembly and quenched to achieve its through hardness requirements. However, when the workpiece was manufactured from a composite structure, it was found that the junction of the component pieces is typically the weakest link in the part and has a finite life less than that which otherwise might be achieved.